1. Field of the Invention
The present invention relates first of all to a method for focusing, on at least one point to be examined of a microwave radiation source, the antennae of an antenna array receiving the radiation from the point with respective reception phase shifts.
Such a method is used when it is desired to obtain, from the microwave radiation coming from an object to be analysed, a microwave image of this object. For this, an antenna assembly is organized so as to form an array, this term being used in a sense close to, but wider than, that which it has in optics, and this antenna array is successively focused on each of the points to be examined of the object, so as to construct point by point the microwave image of this object. Microwave image forming systems have, in particular, applications in the biomedical field for the detection and treatment of tumours, for example, as well as in the civil engineering field, for detecting buried objects for example, or else for testing materials treated by microwave radiation (polymerization, defreezing, drying . . . ). Furthermore, the antennae known under the name of electronic sweep antennae, used for example in radar and telecommunications, are organized in the form of a fixed antenna array which has maximum sensitivity in a variable electronically controllable direction and use such a method corresponding to the focusing on a point situated at an infinite distance, and defined solely by its direction.
2. Description of the Prior Art
A focusing method is already known in which the signal received by each antenna is phase shifted in a microwave phase shifter, the phase shifted signals being added and the summation signal being subjected to microwave detection. The phase law which determines the particular phase shift to be applied to each signal to be received is established so that the contributions from the point on which the array is focused are in phase at the time of summing, the contributions coming from other points having any phases with respect to each other. Thus, after summation, the signal obtained represents principally the contribution of the focusing point.
Now, this known method has the drawback of being difficult to implement from a practical point of view, for it requires the use of as many microwave phase shifters as there are antennae in the array. Now, a microwave phase shifter is a relatively complex component, of a high cost price and requiring considerable space. In the cases where the number of antennae reach several hundreds, even several thousands, this solution is therefore very expensive. In addition, it is clear that the space required by the phase shifters conditions the distance between an antenna and the adjacent antennae, that is to say the pitch of the array. Now, an antenna with too large a pitch will give an image of poor quality. Another drawback of this method is that it does not allow simultaneous focusing on several points at a given time.
To overcome these drawbacks, the use of a synthetic focusing method is known. In such a method, each signal received is not phase shifted but is subjected directly to coherent microwave detection, that is to say detection for knowing the modulus and phase of the detected signal. The modulus and phase of each signal received are then acquired by a computer. The computer carries out digital processing of the set of such data, so as to extract therefrom the contribution coming from any point of the object. Such processing is then tantamount to synthetic focusing as opposed to analog focusing obtained by using microwave phase shifters.
The synthetic focusing method has however the following drawbacks:
the signal to noise ratio is very much less than the signal to noise ratio of the analog focusing method;
the only digital processing algorithms which it is possible to use only correspond to particular array geometries and in a limited number (plane, cylindrical, spherical geometry . . . );
the practical implementation of the method, which requires as many successive coherent microwave detections as there are antennae, involves the use of a microwave multiplexer with a very large number of channels, so of a high cost price and requiring considerable space;
the coherent detections carried out on each signal make the processing of incoherent signals impossible, that is to say signals whose phase varies in a random fashion, such as the microwave signals emitted by the object itself used for example in thermography for obtaining an image of the temperature of the different points of the object;
simultaneous focusing on several points requires the use of as many computers as there are focusing points.
To overcome the drawbacks of the known methods the applicant has sought an analog type focusing method which may be adapted to any array geometry, but not requiring the use of microwave phase shifters as in the only known analog focusing method. For this, the applicant had the idea of carrying out the phase shifts not on the microwave signals but on a low frequency signal.